The present invention relates to a non-aqueous electrolyte cell, and more particularly to an improvement in an anode therefor.
A non-aqueous electrolyte cell which employs an alkali metal such as lithium for an anode active material, and an electrolyte composed of an organic solvent such as propylene carbonate, .gamma.-butyrolactone, dimethoxyethane, tetrahydrofuran, dioxolane, etc. with LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiPF.sub.6, LiCF.sub.3 SO.sub.3, etc. dissolved therein, has an advantage of providing high energy density. Because of this advantage, nowadays non-aqueous primary cells are used in small electronic appliances including watches and cameras, and non-aqueous secondary cells are used in portable apparatus such as portable telephones, portable personal computers, video movie recorders, etc.
The non-aqueous electrolyte cell is also characterized by long shelf life. This is because the alkali metal used as the anode active material reacts easily with electrolyte constituents owing to a high reactivity, thereby forming on the anode surface a passivating film which acts to suppress a reaction of self-discharge.
On the other hand, the alkali metal in a shape of tree branch, needle, fibril or similar appearance, generally known as dendrites, are deposited markedly on the anode surface during charging. This is because the passivating film acts to localize the deposition of the alkali metal in specific areas on the anode surface. Formation and subsequent growth of such dendrites has lead to a problem that the anode and a cathode are short-circuited internally. Furthermore, in a process of discharge, the dendrites are locally dissolved, resulting in discontinuities at various places and it becomes impossible to dissolve all of the alkali metal deposited during charging. This has lead to the further problem that the charge/discharge efficiency drops significantly.
To suppress the formation of such dendrites during charging, a charging method is proposed, for example, in "Progress in Batteries and Solar Cells," Vol. 2, p. 54, 1979, in which, when lithium is used as the alkali metal, an anode potential is maintained higher than -50 mV with respect to an Li.sup.+ /Li electrode and the lithium is deposited under a mild condition at a current density held equal to or less than 0.3 mAh/cm.sup.2. Another method, which is proposed in "Electrochimica Acta," Vol. 3, p. 1715, 1985, involves using a mixture of a high-dielectric-constant substance such as propylene carbonate and a low-viscosity substance such as dimethoxyethane for the solvent of the electrolyte, thereby suppressing the formation of the dendrites while improving the charge/discharge efficiency.
However, the former method involving charge control is inferior in efficiency and consequently requires a long charge time.
On the other hand, when a mixture of the high-dielectric-constant substance and the low-viscosity substance is used for the solvent of the electrolyte, as in the latter method, the formation of the dendrites in charge/discharge cycles is suppressed while retaining a low self-discharge characteristic at room temperature. This method, however, involves problems in terms of characteristics under severe conditions expected in actual usage, such as rapid charge/discharge characteristics, charge/discharge characteristics at high temperatures of 60.degree. C. and above, shelf life at such high temperatures, or charge/discharge characteristics at low temperatures of -20.degree. C. and below. The deterioration of rapid charge/discharge characteristics and the deterioration of various characteristics at high temperatures are both attributed to acceleration of a formation of the dendrites and reaction between the solvent and the alkali metal used as the anode active material. The deterioration of characteristics at low temperatures is due to such factors as the decrease in reaction efficiency.
The present invention is directed to overcome the above-mentioned problems, and to provide a highly reliable anode for a non-aqueous electrolyte cell which has long shelf life and cycle life even under severe environmental conditions.